How can i prevent hypovolemic cardiac arrest?

Drink water: Drink water, don't bleed to death, don't get cholera and not treat it, etc.

Stay hydrated: Hypovolemic cardiac arrest is rare - usually from trauma (alot of blood loss all at once). In general, a person should try to stay "euvolemic" - not too much fluid on board and not too little - dehydration.. But even a fairly dehydrated person is more likely to have cardiac issues from blood electrolyte issues and not from hypovolemia.